Displacement sensors, such as microphones and pressure sensors, are well-known. Displacement sensors based on capacitive, impedance, and optical measurements have been developed. Optical displacement sensors are particularly attractive as they overcome many of the limitations of capacitive and impedance measurement techniques, such as low sensitivity, the need for high voltage biasing, poor electrical isolation, or response nonlinearities.
Many optical displacement sensors known in the prior art operate by detecting light reflected by an optical element that changes its reflectivity in response to a pressure differential, sound, vibration, etc. A Fabry-Perot interferometer has often been used as such an optical element. A Fabry-Perot interferometer is an optical element whose reflectivity depends on the spacing between two parallel partially-reflective surfaces. In order to form a Fabry-Perot interferometer that is sensitive to sound, etc., one surface of the Fabry-Perot interferometer is a surface of a movable membrane. When the movable membrane moves in response to incident sound, for example, the reflectivity of the Fabry-Perot interferometer is changed. The intensity of the detected light changes as well, therefore, thereby resulting in an electrical signal based on the incident acoustic energy of the sound.
The dynamic range of the reflectivity change of the Fabry-Perot interferometer limits the sensitivity of prior art microphones. In addition, noise in the detected signal, such as detector noise or source noise, reduces the fidelity of prior art optical microphones, since detector and/or source noise can be difficult to distinguish from a reflectivity change of the Fabry-Perot interferometer. The ratio of signal strength to noise (i.e., the signal-to-noise ratio (SNR)) is a key performance metric for displacement sensor performance. When used as a hearing aid microphone, for example, low SNR impairs the ability to provide clear high-fidelity reproduction of the sound desired by the hearing aid user. It also impairs the ability to improve hearing aid performance through signal processing techniques.
There exists, therefore, a need for an optical displacement sensor that overcomes some of the limitations of the prior art; specifically, an optical microphone with a higher SNR output signal.